I think this would be the next step for me. For now our performance isn't a big issue and bounces are processed in background cron job once per several minutes. For now volumes of bounces are not so high as you have, so we are just fine with spawning application. Thanks for commenting.
Cool - when we send out our newsletters to users, about 70,000 total active emails, we get a solid 1000 bounces in the first hour or so. Bounces decrease each time we newsletter, since we flag users with a bounce_count, and if it reaches 2, or the bounce is a 5xx (permanent error), we don't email them anymore. But, 1000 full stack loads in an hour on our mailserver would crush it!
